1. Field of the Invention
This application relates generally to the field of wireless communication. In particular, this application relates to modal antennas adapted for diversity applications and methods for designing modal antennas for diversity or other scheme requiring two or more radiation patterns from the same or different location.
2. Related Art
As new generations of handsets and other wireless communication devices become smaller and embedded with increased applications (mobile internet browsing, software downloads, etc.), new antenna designs are required to address inherent limitations of these devices and to enable new capabilities. With classical antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular frequency and with a particular bandwidth. In multi-band applications, more than one such resonant antenna structure may be required. But effective implementation of such complex antenna arrays may be prohibitive due to size constraints associated with mobile devices.
Recent developments in the art have provided for steering of antenna radiation characteristics as is described in commonly owned U.S. patent application Ser. No. 12/043,090, titled “ANTENNA AND METHOD FOR STEERING ANTENNA BEAM DIRECTION”, and filed Mar. 5, 2008; the entire contents of which are hereby incorporated by reference.
More recently, “beam steering antennas” have evolved toward applications for correcting situations where a wireless device may enter a location having little to no signal reception, otherwise known in the art as a “null” or “null field”. When the device enters a null, the beam steering mechanism activates to steer antenna radiation characteristics into a useable state or mode. Thus, these “null steering antennas” have recently been referred to as “active modal antennas”, or simply “modal antennas”, due to the fact that these antennas provide various modes of operation, wherein a distinct radiation pattern exists for each antenna mode of the modal antenna. Antenna modes can be different and exhibit different radiation shapes but could also be configured to show more measured and continuous changes in radiation pattern characteristics.
To further understand the invention, one having skill in the art must be familiar with antenna diversity schemes. In the prior art, antenna diversity generally utilizes two or more antenna radiators in an effort to improve the quality and reliability of a wireless communication link. Often, especially indoors and urban canyons, the line of sight between a transmitter and receiver becomes saturated with obstacles such as walls and other objects. Each signal bounce may introduce phase shifts, time delays, attenuations, and distortions which ultimately interfere at the receiving antenna. Thus, destructive interference in the wireless link is often problematic and results in a reduction in performance.
Antenna diversity schemes can mitigate interference from multipath environments by providing multiple antennas to the receiver, and therefore multiple signal perspectives. Each of multiple antennas within a diversity scheme experiences a distinct interference characteristic. Accordingly, at a physical location where a first antenna may experience a null—the second antenna is likely to receive an effective signal. Collectively, the diversity scheme provides a robust link.
Antenna diversity can be implemented generally in several forms, including: spatial diversity; pattern diversity; polarization diversity; and transmit/receive diversity. Although each form is distinct, many antenna systems can be designed according to multiple forms.
Spatial diversity generally includes multiple antenna radiators having similar characteristics. The multiple antennas are physically spaced apart from one another. Where a first antenna may experience a significant reduction in signal reception, i.e. a null, a second antenna is adapted for use with the receiver.
Pattern diversity generally includes two or more co-located antennas with distinct radiation patterns. This technique utilizes directive antennas that are usually separated by a short distance. Collectively, these co-located antennas are capable of discriminating a large portion of angle space and may additionally provide relatively higher gain with respect to an omni-directional antenna element.
Polarization diversity generally includes paired antennas with orthogonal polarizations. Reflected signals can undergo polarization changes depending on the medium through which they are traveling. By pairing two complimentary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fade.
Transmit/Receive diversity generally includes the ability to provide diversity for both transmit and receive functions. Implementing transmit diversity can be more problematic due to the need for input from the base station or end side of the communication link regarding link performance.
Each of the above diversity schemes requires one or more processing techniques to effectuate antenna diversity, such as: switching, selecting, and combining. Switching is the most power-efficient processing technique which generally includes receiving a signal from a first antenna until the signal level fades below a threshold level, at which point a switch engages the second antenna radiator for communication with the receiver. Selecting is a processing technique which provides a single antenna signal to the receiver; however the selecting process requires monitoring of signal to noise ratio (SNR) or similar quantification for determining the ideal signal for utilization by the receiver. Combining is a processing technique wherein each of multiple signals are weighted and combined into an output signal for communication with the receiver. Although these techniques have been described for reception, their analogs are possible for transmit functions. Furthermore, a combination of these techniques is possible for dynamic diversity control.
Examples of prior art antenna diversity schemes can be recognized in FIGS. 1(a-b). FIG. 1a represents an architecture with two receive chains (two radiators) illustrating a minimum mean squared error (MMSE) combining technique. Here, the signal is weighted at each path and chosen to provide a minimum mean square between combined voltages. Alternatively, FIG. 1b represents an antenna architecture with two radiators for maximum ratio combining (MRC) processing. Here, a weighting factor is applied to each receive signal.
Although the above-described antenna diversity schemes can be implemented to provide a robust signal link, there are disadvantages associated with these current diversity architectures. For instance, size constraints can be significantly limited with multiple antenna radiators and coupling with nearby electronics of a communications device is a common problem with antenna system design. Additionally, power limitations and efficiency can be a problem in many instances where multiple paths are energized. Implementing 3 or more receive diversity or transmit diversity antennas only amplifies the problems related to volume required for additional antennas as well as circuit board area required for antennas and transmission lines. Receivers become more complicated as additional receive ports are implemented to accommodate larger numbers of antennas.
There has yet to be suggested in the art a diversity antenna scheme made up of a single antenna. In fact, prior art diversity architectures require two or more antenna radiators. With the advent of active modal antennas, the applicants herein disclose a single-antenna scheme for diversity and related applications, thereby providing a robust antenna with significantly reduced size for enabling compact wireless devices, inter alia.